A gas which is as homogeneous as possible is conventionally fed to the compressor of gas turbines for compression. In applications in which gases with different composition are provided, these are first mixed and then fed to the compressor. For example, in the case of recirculation of flue gases, use is made of a flue gas with a gas composition which differs from that of fresh air. Recirculation is a technology which can basically be used in gas turbines for a wide variety of purposes, for example for controlling emissions, for reducing the flue gas volume, for carbon dioxide separation, etc. During the recirculation of flue gases in a gas turbine, a significant proportion of the flue gas is branched from the overall flue gas flow and, after cooling and scrubbing, is normally fed again to the intake mass flow of the turbine or to the compressor of the turbine. The flue gas composition differs appreciably from the composition of fresh ambient air. The recirculated flue gas flow is conventionally mixed with fresh air from the environment and this mixture is then fed to the compressor.
By means of flue gas recirculation, the carbon dioxide partial pressure in the flue gases can advantageously be increased in order to reduce power losses and efficiency losses of power plants with carbon dioxide separation. In addition, flue gas recirculation has been proposed with the aim of reducing the oxygen content in the intake gases of gas turbines in order to thereby reduce NOx emissions.
In the prior art, in order to ensure satisfactory combustion in the case of low oxygen content of the combustible gases, the fresh air is mixed with the recirculated flue gases as thoroughly as possible in order to provide a homogeneous gas mixture at the combustion chamber inlet. A mixer which is suitable for low pressure loss mixing of fresh air with recirculated flue gases is known from WO2010/142573 A2, for example.
Depending upon operating conditions, stability problems, especially pulsations (also referred to as thermoacoustic vibrations) can occur during combustion in a gas turbine with premix burners. In order to reduce pulsations, it is known from WO2005095864 to supply individual burners with fuel in a throttled manner, as a result of which the premix burners in question have a combustion temperature which differs from the combustion temperature of all the other unthrottled premix burners. This leads to an asymmetry in the temperature distribution of the premix burner arrangement, as a result of which the development of pulsations inside the combustion chamber can be effectively counteracted.
The throttled burners operate at a hot gas temperature which is reduced compared with the average hot gas temperature, but which can lead to incomplete combustion. In addition, the unthrottled burners operate at an increased hot gas temperature, which can lead to higher NOx emissions and can have a negative effect upon the service life.
Incomplete combustion can lead to increased CO emissions (carbon monoxide emissions), especially in plants with flue gas recirculation, in which the intake gases of the gas turbine have a reduced oxygen content compared with fresh air.